The field of the present invention is computed tomography and, particularly, computer tomography (CT) scanners used to produce medical images from x-ray attenuation measurements.
As shown in FIG. 1, a CT scanner used to produce images of the human anatomy includes a patient table 10 which can be positioned within the aperture 11 of a gantry 12. A source of highly columinated x-rays 13 is mounted within the gantry 12 to one side of its aperture 11, and one or more detectors 14 are mounted to the other side of the aperture. The x-ray source 13 and detectors 14 are revolved about the aperture 11 during a scan of the patient to obtain x-ray attenuation measurements from many different angles through a range of at least 180.degree. of revolution.
A complete scan of the patient is comprised of a set of x-ray attenuation measurements which are made at discrete angular orientations of the x-ray source 13 and detector 14. Each such set of measurements is referred to in the art as a "view" and the results of each such set of measurements is a transmission profile. As shown in FIG. 2A, the set of measurements in each view may be obtained by simultaneously translating the x-ray source 13 and detector 14 across the acquisition field of view, as indicated by arrows 15. As the devices 13 and 14 are translated, a series of x-ray attenuation measurements are made through the patient and the resulting set of data provides a transmission profile at one angular orientation. The angular orientation of the devices 13 and 14 is then changed (for example, 1.degree.) and another view is acquired. An alternative structure for acquiring each transmission profile is shown in FIG. 2B. In this construction, the x-ray source 13 produces a fan-shaped beam which passes through the patient and impinges on an array of detectors 14. Each detector 14 in this array produces a separate attenuation signal and the signals from all the detectors 14 are separately acquired to produce the transmission profile for the indicated angular orientation. As in the first structure, the x-ray source 13 and detector array 14 are then revolved to a different angular orientation and the next transmission profile is acquired.
As the data is acquired for each transmission profile, the signals are filtered, corrected and digitized for storage in a computer memory. These steps are referred to in the art collectively as "preprocessing" and they are performed in real time as the data is being acquired. The acquired transmission profiles are then used to reconstruct an image which indicates the x-ray attenuation coefficient of each voxel in the reconstruction field of view. These attenuation coefficients are converted to integers called "CT numbers", which are used to control the brightness of a corresponding pixel on a CRT display. An image which reveals the anatomical structures in a slice taken through the patient is thus produced.
The reconstruction of an image from the stored transmission profiles requires considerable computation and cannot be accomplished in real time. The prevailing method for reconstructing images is referred to in the art as the filtered back projection technique, and the calculating time required when using this technique is determined in part by the amount of attenuation data acquired during each view, or transmission profile. In particular, the filtering step in this technique is carried out using a Fourier transformation, and the calculating time for this transformation can be affected dramatically with a change in the amount of acquired transmission profile data.
Referring to FIG. 3, the proper reconstruction of an image requires that the x-ray attenuation values in each view pass through all of the objects located in the aperture 11. If the object is larger than the acquired field of view, it will attenuate the values in some transmission profiles as shown by the vertically oriented view in FIG. 3, which encompasses the supporting table 10, and it will not attenuate the values in other transmission profiles as shown by the horizontally oriented view in FIG. 3. As a result, when all of the transmission profiles are back projected to determine the CT number of each voxel in the reconstructed field of view, the CT numbers will not be accurate. This inaccuracy can be seen in the displayed image as background shading which can increase the brightness or darkness sufficiently to obscure anatomical details.
The solution to this problem, of course, is to insure that the entire object in the aperture 11 is within the field of view of the acquired data. For example, when imaging the patient's head, a head holder 15 such as that disclosed in U.S. Pat. No. 4,400,820 is employed and extends from the end of the table 10 and closely follows the contour of the head. The head holder 15 supports the patient, but does not significantly increase the size of the field of view required to encompass it from all angles.
Unfortunately, there are many instances in which it is not possible to confine all objects to the field of view. For example, it may not be possible to use the head holder on trauma patients, in which case, the table 10 must be employed for support and will reside in the aperture 11. In such cases, either the consequent degradation of image quality must be accepted, or the field of view of the acquired data used in making the reconstructed image must be expanded with the consequent increase in computation time.